Resistor



N. VASILEFF Sept. 30, 1947.

RESISTOR Filed Dec. 4, 1945 FIG.2.

FIG.3.

Has.

LENGTH OF RESISTOR INVENTOR. NATCHO VASILEFF FIG].

Patented Sept. 30, 1947 UNITED STATES PATENT OFFICE 4 Claims.

The invention described herein maybe manufactured and used by or for theGovernment for governmental purposes, without the payment to me oi anyroyalty thereon.

This invention relates to resistors,-and is, particularly valuable asincorporated in :power,.wire wound resistors. a v

An object of the invention is to provide a power resistor which will be,and remain, moisture proof.

Another object is to provide a power resistor which will operate at alower maximum temperature for a given rating. 1

A further object is to provide a resistor having a more uniformtemperature" distribution.

A still further object is to'provi de a power resistor having aninsulator which will remain intact, and not crack and break ofL-andwhich will also meet the requirements for {class 1, grade 1 resistors.

Other objects and advantages will be appreciated by those skilled in theart from a reading of the specification.

Power resistance'elements, as resistance wires of class 1 resistors, areinsulated with vitreous enamels. These enamels crack and pieces breakoff, due to the different coeilicients of expansion of the core, theresistance wire and the insulation, leaving the insulation defective.Furthermore, these enamels have alkaline constituents which, in thepresence of moisture, directly and in directly corrode, the resistancewire, the indirect corrosion resulting from electrolysis of thedissolved alkaline substance. The latter has access to the wire throughthe mentioned cracks, and at the bare spots resulting from pieces of thecracked enamel falling of! the wire. The corrosion of the wire obviouslyalters its resistance and eventually destroys the wire, so hot spotsdevelop at points of corrosion, where the effective dimeter isdiminished.

The usual power resistor has a core of some insulating material, such asporcelain or steatite for example. These materials have poor heatconductivity, so that resistance elements wound thereon operate at arelatively high temperature. The high operating temperature requiresthat an inorganic substance, as an enamel, be employed as an insulatorcovering the resistance element. The inorganic insulators are brittleand, as stated above, crack, and break off in parts,

The new combination employed in the resistor amended April 30, 1928; 3700. G. 757) 2 of this invention comprises a resistance element insulatedwith a silicone resin, and an aluminum oxide core on'which theresistance element is wound. Thesilicone resin withstands a temperatureabout C. higher than the usual insulating varnishes do. However, theseresins do not withstand the rated temperature of 275, C. specified forclass 1 resistors. The core of aluminum oxide is therefore used toreduce the operating temperature to ,a point allowing the use of asilicone resin, this oxide having a sufficiently high specific heatconductivity to accomplish the purpose, as distinguished from the usualporcelain, steatite, etc. Aluminum oxide in fact has a specific heatconductivity comparable to that of certain metallic alloys, such asGerman silver, for example.

The invention is illustrated in the accompanying drawings, in whichFigure 1 is a plan view of an aluminum oxide core used in the resistor;

Figure 2'is an end view of Figure 1;

Figure 3 is a plan' view of a length of wire, enlarged in diameter, andcoated with a silicone resin;

Figure 4 is an end view of Figure 3;

Figure 5 is an elevational view of a class 1, grade 1, power resistorembodying the invention;

Figure 6 is an end view of Figure 5; and

Figure 7 shows comparative graphs of the temperature distribution of theusual power resistor and of the present resistor.

Referring to the drawings for a more detailed description thereof,Figure 1 shows a hollow cylindrical core iii of aluminum oxide, on whicha, resistance element is to be Wound. Such a core is used for itsinsulating quality and also for its high heat conductivity, as explainedabove. The later property rapidly dissipates the heat generated in theresistance element, resulting in a more nearly uniform temperature alongthe length of the core and a, lower maximum and average temperature, themaximum being low enough to allow the use of the desirable siliconeresin.

Figure 3 shows a length I! of Nichrome or other resistance wire which iscovered with a silicone resin M, the wire with its coating beingdesignated by the numeral IS. The insulated resistance wire I 5 is woundhelically on the core in as shown in Figure 5, and the wire-wound coreis mounted on the usual type of uprights I! which are secured to, andrise from, a base IS. The whole assembly may desirably be immersed in asilicone resin to receive a protective coating thereof.

Figure 7 shows curves 20 and 2! in which temperatures, in degreescentigrade, are plotted against positions along the length of theresistor. Curve 20 shows the temperature distribution in the usuallength of the resistor of the type having a porcelain or steatite core,with enamel insulation on the resistance element. Curve 20 shows thistype of resistor as having a maximum temperature of 275 C., which is atthe mid point of the resistor, from which point the temperature falls to160 C. at the ends, with a spread or dillerence of 115 C.

Curve 2| shows the temperature distribution along the resistor of myinvention. The curve indicates a maximum of 230 C. at the middle of theresistor, which temperature is 45 C. lower than the midpoint of curve20, due to the aluminum oxide core having better heat conductivity thanother types of core. The operatmg temperature is thereby brought down tothe point where a silicone resin may be used, the temperature of 275 C.of prior resistors being too high for these resins to withstand. Curve21 also indicates a temperature of 190 at the ends of the resistanceelement, with a spread or difference, therefore, of 40 C. between themid point and ends, as against a difference of 115 between the mid pointand ends of prior power resistors.

It will be readily appreciated from the foregoing matter that the newresistor has accomplished the objects of the invention and is asubstantial advance in the art.

What s claimed is:

1. A resistor comprising a core of aluminum oxide, a resistance elementwound on the core, and an insulating coating, consisting of a siliconeresin, applied directly to the resistance element.

2. A resistor comprising a core of aluminum oxide, a resistance elementhelicaily wound on the core, and an insulating coating, consisting of asilicone resin, applied directly to the surface of the resistanceelement.

3. A resistance element and an insulating coating, consisting of asilicone resin, applied directly to the resistance element.

4. A metallic resistance wire and an insulating coating, consisting of asilicone resin, applied directly to the resistance element.

NATCHO VASILEFF.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,035,707 Oldbury Mar. 31, 19362,372,212 Lewin Mar. 27, 1945 2,377,600 Barker et a1 June 5, 19452,397,568 Seaman Apr. 2, 1946

